1. Field of the Invention
The present invention relates to dielectric resonant devices, such as dielectric filters and dielectric duplexers, including dielectric blocks provided with conductive films disposed on the inner and outer surfaces thereof. The present invention also relates to a communication apparatus using the dielectric resonant devices, and to a method for forming an input-output electrode of a dielectric resonant device.
2. Description of the Related Art
FIG. 5 is a perspective view of a known dielectric filter using a dielectric block. FIG. 5 shows a rectangular-parallelepipedal dielectric block 1. The dielectric block 1 is provided with inner conductor holes 2a and 2b extending from a surface A to the opposing surface. Each of the inner conductor holes 2a and 2b includes an inner conductor on the inner surface thereof. The dielectric block 1 is provided with an outer conductor 4 on the five surfaces other than the surface A. The inner conductors are connected to the outer conductor at the surface opposing the surface A. The inner conductors have open ends at the surface A, and are connected to each other at the opposing surface. The dielectric block 1 includes input-output electrodes 5a and 5b on the outer surfaces, which are separated from the outer conductor 4.
In the known dielectric filter shown in FIG. 5, the input-output electrodes 5a and 5b are formed by providing regions of the outer conductor separated from the other regions thereof by cutting a part of the outer conductor at predetermined regions by a cutting tool for cutting a conductive film. The method is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 6-310911.
In the dielectric filter prepared by the above-described method, an external Q value (Qe) is determined based on the electrostatic capacitance (external coupling capacitance) between the input-output electrodes and the adjacent inner conductors.
That is, the size of the input-output electrodes is determined according to the characteristics of the dielectric filter to be obtained, which vary according to the external coupling capacitance. In a known method, a cutting tool is required, in which cutting edges are prepared for cutting the conductive film according to each external coupling capacitance to be obtained.
In the known method, therefore, tools having cutting edges in various shapes must be prepared according to each external coupling capacitance desired, resulting in tools having very limited application. Moreover, since the pattern of the input-output electrodes is determined by the position at which the edge of the tool is applied to the outer surface of the dielectric block, the pattern of the input-output electrodes changes as the tool wears out, which causes a problem in that the external coupling capacitance changes from the desired value.
A dielectric filter is proposed, as disclosed in U.S. Pat. No. 5,162,760, in which a side of each input-output electrode is disposed at an open surface of a dielectric block. With this arrangement, desired input-output electrodes can be formed by partially cutting an outer conductor provided on the outer surfaces of the dielectric block.
However, in the dielectric filter disclosed in the above-described U.S. Patent, the following problems occur.
Firstly, electrostatic capacitance between the input-output electrodes and the inner conductors cannot be large because the input-output electrodes are disposed only on a surface of the dielectric block opposing a mounting board (on a bottom surface).
Secondly, because the input-output electrodes are disposed only on a surface of the dielectric block opposing the mounting board (on the bottom surface), isolation cannot be sufficiently ensured due to a short distance between the input-output electrodes provided in a miniaturized dielectric filter.
Thirdly, input-output electrodes on the mounting board cannot be standardized because a pattern of the input-output electrodes on the mounting board must be changed according to the pattern of the input-output electrodes of the dielectric filter which changes according to the area thereof required for the external coupling capacitance determined based on the characteristic of the dielectric filter.
Fourthly, the inspection of a soldered state of the input-output electrodes is difficult because their soldering fillets are difficult to see due to their position, which is the surface of the dielectric block (the bottom surface) opposing the mounting board.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric resonant device, a dielectric filter, a dielectric duplexer, and a communication apparatus using the same, in which a desired external coupling capacitance is variably obtained by using one type of cutting tool for cutting a conductive film.
Further, preferred embodiments of the present invention provide a method for forming input-output electrodes of the dielectric resonant device to obtain a given external coupling capacitance.
In accordance with one preferred embodiment of the present invention, a dielectric resonant device is provided which comprises a dielectric block in a substantially rectangular-parallelepipedal shape, including at least one inner conductor therein; and an outer conductor provided on the outer surfaces of the dielectric block except at least one outer surface being an open surface and substantially perpendicular to the axis of the inner conductor. At least one input-output electrode is provided in contact with the open surface to extend over two outer surfaces of the dielectric block adjacent each other, the two outer surfaces being parallel to the axis of the inner conductor.
In the dielectric resonant device according to the invention, the input-output electrode may be formed to be separated from the outer conductor by providing a groove on the surface of the dielectric block.
By providing the input-output electrodes in contact with the open surface, the length of each of the input-output electrodes along the axis of the inner conductor can be determined by using the open surface as a longitudinal end of the input-output electrode and by determining only the position of the other end thereof, thereby enabling the area of a desired pattern of the input-output electrode to be determined. Moreover, maximum electrostatic capacitance between the input-output electrodes and the inner conductors can be ensured to be large by extending the input-output electrodes to an edge of the dielectric block. When miniaturizing the device, a distance between the input-output electrodes disposed on a mounting surface can be kept sufficiently large for maintaining isolation between the input-output electrodes. External coupling capacitance is determined by the area of the input-output electrodes which extend from a mounting surface to the surfaces adjacent thereto of the dielectric block. Therefore, the area of the input-output electrodes can be changed in accordance with a desired external coupling capacitance without changing the pattern thereof on the mounting surface so that input-output electrodes on a mounting board opposing the input-output electrodes on the mounting surface can be standardized in their pattern. With this arrangement, soldering fillets thereof can be checked after the device is mounted on a mounting board.
According to another preferred embodiment of the present invention, a dielectric filter is provided comprising a dielectric resonant device described above, wherein the input-output electrode is provided at two positions, one of the input-output electrodes serving as an input port and the other one of the input-output electrodes serving as an output port. With this arrangement, the input port and the output port of the dielectric resonant device functions as a dielectric filter having a band-pass characteristic.
According to yet another preferred embodiment of the invention, a dielectric duplexer is provided comprising a dielectric resonant device described above, wherein the input-output electrode is provided at three positions, the three input-output electrodes respectively serving as a transmission signal input port, a receiving signal output port, and a transmission-receiving signal input-output port. With this arrangement, the dielectric resonant device may function as a dielectric duplexer (an antenna duplexer), in which the transmission signal input port inputs a transmission signal from a transmission circuit, the transmission-receiving signal input-output port outputs a transmission signal to an antenna and inputs a received signal from the antenna, and the receiving signal output port outputs a received signal to a receiving circuit.
According to yet another preferred embodiment of the present invention, a communication apparatus is provided comprising the dielectric filter and the dielectric duplexer described above. The communication apparatus having excellent high-frequency circuit characteristics includes the dielectric filter or the dielectric duplexer in which desired characteristics are obtainable by setting an external coupling capacitance between the input-output electrodes and the inner conductors at an optimal value. The cost of the communication apparatus may be decreased by using the dielectric filter or the dielectric duplexer which can be produced at a low cost.
According to yet another preferred embodiment of the present invention, a method for forming an input-output electrode of a dielectric resonant device is provided, comprising: providing an inner conductor in a substantially rectangular-parallelepipedal dielectric block; providing an outer conductor on the outer surfaces of the dielectric block except at least one outer surface being an open surface perpendicular to the axis of the inner conductor; and forming an input-output electrode by cutting surfaces of the dielectric block with a cutting tool including cutting edges applied to two outer surfaces of the dielectric block adjacent each other, which are disposed substantially in parallel to the axis of the inner conductor, the cutting edges applied to the two outer surfaces along two first lines, one line on each of the two outer surfaces, extending from the open surface substantially in parallel to the axis of the inner conductor, thereby removing a conductive film along the two first lines, and the cutting tool including a cutting edge applied to the two outer surfaces along a second line connecting the two first lines over the two outer surfaces, thereby removing a conductive film along the second line.
In the above described method, the cutting edges of the cutting tool may be formed in the same length as the maximum length desired of conductive film removing lines extending in parallel to the axis of the inner conductor from the open surface of the dielectric block, whereby the applied position of the cutting tool to the two adjacent outer surfaces of the dielectric block may be selected along the longitudinal direction of the input-output electrodes to form the conductive film removing lines and to provide input-output electrodes extending from the open surface in any desired length. Namely, the external coupling capacitance may be obtained as desired, according to the applied position of the cutting tool to the dielectric block.